Various dehydrogenation processes have been proposed to dehydrogenate non-aromatic six membered ring compounds. These dehydrogenation processes are typically used to convert non-aromatic compounds such as cyclohexane into aromatic compounds such as benzene wherein the aromatic compound produced may be used as a raw material in a subsequent process. Alternatively, the aromatic compound produced may be used as a raw material in the same process which produced the non-aromatic compound to be dehydrogenated. For example, the dehydrogenation of cyclohexane to benzene can be important in the hydroalkylation process for producing cyclohexylbenzene as illustrated below.
Cyclohexylbenzene can be produced from benzene by the process of hydroalkylation or reductive alkylation. In this process, benzene is heated with hydrogen in the presence of a catalyst such that the benzene undergoes partial hydrogenation to produce a reaction intermediate such as cyclohexene, which then alkylates the benzene starting material. In this regard, U.S. Pat. No. 6,037,513 has disclosed that cyclohexylbenzene selectivity in the hydroalkylation of benzene can be improved by contacting the benzene and hydrogen with a bifunctional catalyst comprising of at least one hydrogenation metal and a molecular sieve of the MCM-22 type. The hydrogenation metal is preferably selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof and the contacting step is conducted at a temperature of about 50° C. to 350° C., a pressure of about 100 kPa to 7000 kPa, a hydrogen to benzene molar ratio of about 0.01 to 100 and a weight hourly space velocity (WHSV) of about 0.01 hr−1 to 100 hr−1. This reference discloses that the resultant cyclohexylbenzene can then be oxidized to the corresponding hydroperoxide and the peroxide decomposed to the desired phenol and cyclohexanone. Nonetheless, notwithstanding the high selectivity of the hydroalkylation reaction over the MCM-22 molecular sieve-based bifunctional catalyst in the disclosed method, certain amounts of cyclohexane and methylcyclopentane are produced in the hydroalkylation effluent.
Production of impurities such as cyclohexane and methylcyclopentane represent loss of valuable benzene feed. The overall benzene conversion rates are typically only 40 wt % to 60 wt %. Therefore, recycle of unreacted benzene is desired. Unless removed, these impurities including cyclohexane and methylcyclopentane will tend to build up in the recycle stream thereby displacing benzene and increasing the production of undesirable by-products. Thus, a significant problem facing the commercial application of cyclohexylbenzene as a phenol precursor is removing the cyclohexane in the benzene recycle streams.
One solution to this problem is proposed in U.S. Pat. No. 7,579,511, which describes a process for making cyclohexylbenzene in which benzene undergoes hydroalkylation in the presence of a hydroalkylation catalyst to form a first effluent stream containing cyclohexylbenzene, cyclohexane, methylcyclopentane, and unreacted benzene. The first effluent stream is then separated into a cyclohexane/methylcyclopentane-rich stream, a benzene-rich stream, and a cyclohexylbenzene-rich stream and the cyclohexane/methylcyclopentane-rich stream is contacted with a second, low acidity, dehydrogenation catalyst to convert at least a portion of the cyclohexane to benzene and at least a portion of the methylcyclopentane to linear and/or branched paraffins and form a second effluent stream. The benzene-rich stream and the second effluent stream can then be recycled to the hydroalkylation step. However, one problem with this process is that cyclohexane and methylcyclopentane have similar boiling points to that of benzene so that their separation by conventional distillation is difficult.
Another solution is proposed in International Patent Publication No. WO2009/131769, in which benzene undergoes hydroalkylation in the presence of a hydroalkylation catalyst to produce a first effluent stream containing cyclohexylbenzene, cyclohexane, and unreacted benzene. The first effluent stream is then divided into a cyclohexylbenzene-rich stream and a C6 product stream comprising cyclohexane and benzene. At least part of the C6 product stream is then contacted with a second catalyst under dehydrogenation conditions to convert at least part of the cyclohexane to benzene and produce a second effluent stream which comprises benzene and hydrogen and which can be recycled to the hydroalkylation step.
Both of the processes disclosed in U.S. Pat. No. 7,579,511 and WO2009/131769 rely on the use of a dehydrogenation catalyst comprising a Group 8, 9, or 10 metal on a porous inorganic support such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, activated carbon, and combinations thereof. WO11/096991 (2010EM139) further discloses a dual-component catalyst system for the dehydrogenation reaction.